The power generation research and development community faces an important challenge in the years to come: to produce increased amounts of energy under the more and more stringent constraints of increased efficiency and reduced pollution. The increasing costs associated with fuel in recent years further emphasize this mandate.
Gas turbines offer significant advantages for power generation because they are compact, lightweight, reliable, and efficient. They are capable of rapid startup, follow transient loading well, and can be operated remotely or left unattended. Gas turbines have a long service life, long service intervals, and low maintenance costs. Cooling fluids are not usually required. These advantages result in the widespread selection of gas turbine engines for power generation. A basic gas turbine assembly includes a compressor to draw in and compress a working gas (usually air), a combustor where a fuel (i.e., methane, propane, or natural gas) is mixed with the compressed air and then the mixture is combusted to add energy thereto, and a turbine to extract mechanical power from the combustion products. The turbine is coupled to a generator for converting the mechanical power generated by the turbine to electricity.
A characteristic of gas-turbine engines is the incentive to operate at as high a turbine inlet temperature as prevailing technology will allow. This incentive comes from the direct benefit to both specific output power and cycle efficiency. Associated with the high inlet temperature is a high exhaust temperature which, if not utilized, represents waste heat dissipated to the atmosphere. Systems to capture this high-temperature waste heat are prevalent in industrial applications of the gas turbine.
Examples of such systems are cogeneration systems and combined cycle systems. In both systems, one or more heat exchangers are placed in the exhaust duct of the turbine to transfer heat to feed-water circulating through the exchangers to transform the feed-water into steam. In the combined cycle system, the steam is used to produce additional power using a steam turbine. In the cogeneration system, the steam is transported and used as a source of energy for other applications (usually referred to as process steam).
A prior art cogeneration system typically includes a gas turbine engine, a generator, and a heat recovery steam generator. As discussed earlier, the gas turbine engine includes a compressor, a combustor (with a fuel supply), and a turbine. A compressor operates by transferring momentum to air via a high speed rotor. The pressure of the air is increased by the change in magnitude and radius of the velocity components of the air as it passes through the rotor. Thermodynamically speaking, the compressor transfers mechanical power supplied by rotating a shaft coupled to the rotor to the air by increasing the pressure and temperature of the air. A combustor operates by mixing fuel with the compressed air, igniting the fuel/air mixture to add primarily heat energy thereto. A turbine operates in an essentially opposite manner relative to the compressor. The turbine expands the hot and pressurized combustion products through a bladed rotor coupled to a shaft, thereby extracting mechanical energy from the combustion products. The combusted products are exhausted into a duct. Feed-water is pumped through the steam generator located in the duct where it is evaporated into steam. It is through this process that useful energy is harvested from the turbine exhaust gas. The turbine exhaust gas is expelled into the atmosphere at a stack.
Due to deregulation of the energy market and volatility in energy prices, many cogeneration operators prefer to have the option of shutting down the turbine assembly while retaining the steam generation capability of the cogeneration system (known as fresh air mode operation). To enable operation of this fresh air mode, a furnace is disposed in the exhaust duct. The furnace provides an alternate source of hot gas for steam generation. To increase the efficiency of the fresh air mode, a portion of the exhaust gas may be recirculated back to the furnace. Generally, the efficiency of the fresh air mode increases with an increase in recirculation rate of the exhaust gas. Heat energy lost through the stack also decreases with an increase in recirculation rate of the exhaust gas. However, with the increase of the recirculation rate of exhaust gas, the oxygen concentration at the inlet of the furnace decreases, which, eventually adversely affects combustion stability (of the mixture in the furnace) and generates pollutants. Thus, maintaining stable combustion at the high recirculation rates of exhaust gas is problematic.